Abstract: According to the rules, as a outburst mine with big mining depth, Jiulong Mine must give priority to use of mining protective seam as regional outburst prevention measures, on-site investigation about the related parameters of the protective layer in the process of lower protective layer mining is carried out to determine the scope of the lower protective layer mining. According to the technical conditions of Jiulong Coal Mine protective layer mining, we designed investigation program of the parameters of the protective seam mining, and researched about changes of gas pressure of the protected layer along with the process of lower protective seam mining, accordingly, defined the pressure relief angle in the direction of inclination and direction of protective layer in investigation area, delimited the effective scope of protective layer, and then investigated protection effect in the process of lower protective layer mining.